Making Science Accessible

Natural Disasters – Part 1

Natural disasters. I won’t even make a joke about this, because there are some serious calamities happening right now. As of writing this article, Hurricane Harvey has finished its rampage on Eastern Texas and parts of Central America, and the deadlier Hurricane Irma has brought catastrophic winds and floods to Puerto Rico, Antigua, the Dominican Republic, the Virgin Islands, and Florida. Concurrently, wildfires reign across Idaho, Washington, Oregon, and California. And on the other side of the world, unprecedented flooding from heavy monsoons has killed 1,200 people in Nepal, Bangladesh, and Northern India.

For years, scientists have warned us about the link between the increased frequency of severe weather events and climate change. Now, we are not saying that the recent disasters are directly caused by climate change, however, the scientific community is saying that events like Hurricane Irma will become more likely and more severe if greenhouse gas emissions continue unabated. But what is the relationship between greenhouse gases and these crazy natural disasters? How do scientists determine a link between global warming and dramatic weather events? We’ll take a crack at answering these questions, starting with hurricanes and monsoons, then moving on to wildfires and droughts, before having a short discussion on the potential future impacts of these severe weather events. For the sake of brevity, we will be splitting this topic into two articles – one on hurricanes and monsoons, and another on droughts and wildfires.

Hurricanes, Monsoons, and Floods

Before we start, it would be helpful if we were on the same page. A previous article of ours details The Greenhouse Effect, the process by which greenhouse gases trap heat in the atmosphere, warming up the planet. But, we didn’t really talk about why it matters. Who really cares if the planet warms up a couple degrees? How does climate change affect storm patterns? Long story short, warmer temperatures mean more water from the ocean is evaporating, and warmer air can hold more moisture. Specifically, the air can hold 7% more moisture per degree Celsius it is warmed. This number is determined by the Clausius-Clapeyron equation, a centuries-old physical law which relates temperature, vapor pressure, and enthalpy (total heat in a system) into one mathematical formula. This fancy equation allows us to calculate the vapor pressure of water at one temperature, given that we know the current vapor pressure and the current temperature.

Atmospheric water vapor basically acts as fuel for intense storms, so increasing the atmospheric temperature increases the amount of water vapor that can be held by the air, which could increase the strength of storms. The reason for this is due to air pressure differentials, and the ways warm and cool air behave. Warm, moist air rises up through the atmosphere, and in doing so, reduces the amount of air present near the surface. This creates an area of low pressure, causing air from the surrounding, higher pressure areas to move into the low pressure area. This new air moving in to equalize the pressure difference also becomes warm and moist, and rises up as well, creating a cycle of moving air. After the air rises up several miles, it cools and begins to sink. The moisture in the cool air forms thick clouds. The entire system of cycling air grows in intensity, fueled by the heat and moisture near the surface. This process is detailed in the diagram below.

As the planet warms, more water will heat up and evaporate, and warmer air will be able to hold more moisture. This increases the amount of “fuel” available for hurricanes, making them more destructive.

It is important to note that while climate change may increase the severity of storms, increasing global temperatures may also decrease the number of storms in general. The reason for this decrease in storm frequency is that global warming affects water evaporation rates in various parts of the world in different ways. For example, around the equator, conditions are already hot and humid, so the change isn’t expected to be dramatic. It’s a different story in the colder, drier, polar regions of the world, where even small amounts of heat and evaporation could increase temperatures by significant margins. This would cause the temperature difference between the poles and the equator to decrease. Storms generally form when colder air masses meet and interact with warm, moist air masses, creating clouds and precipitation. But when the difference in temperature between warm and cold air fronts is reduced, storm formation is less likely. In sum, climate change could reduce the frequency of storms in general, but warmer waters and increased atmospheric water vapor makes powerful storms far more likely (i.e. any storms that do end up forming would be more powerful than usual).

Monsoons form in a similar manner to hurricanes – mainly, they are dependent on air pressure differences that result from warm air rising up through the atmosphere. While hurricane formation usually occurs entirely over the ocean, monsoon development requires both land and sea. Land tends to be warmer than the ocean, causing the air above land to be warmer than air above the sea. During the hotter months, warm air over land rises, causing an area of low air pressure. Cooler, moisture-laden air from the sea blows in to equalize this pressure difference. Once this air blows over land, it warms and rises up as well, creating a cycle similar to the hurricane formation detailed earlier. Once the warm, water-saturated air rises a few miles, it cools, losing its ability to retain water (remember, we discussed earlier how warmer air holds water more effectively), thus causing precipitation to fall over land. This cycle is shown in the diagram below.

During the cooler months, the above cycle is reversed, as the land cools more quickly than the ocean, causing air to flow from the land to the ocean. This results in more rains over the ocean. While summer monsoons tend to cause flooding due to the large volumes of water being dumped on land, winter monsoons are likely to cause droughts, because moisture is essentially being transferred from the land to the sea.

As the planet’s average temperature increases, land masses will heat up faster than oceans (mainly because water has a high specific heat capacity). In areas that are affected by monsoons, like South Asia, the increased temperatures over land will accelerate the monsoon process shown above, causing heavier rains and greater flooding during the summer, and more intense droughts in the winter.

To summarize, we discussed the processes by which storms, hurricanes, and monsoons form, and how these processes will be affected by a changing climate in the coming decades. In Part 2 of this topic, we will change gears into wildfires and droughts, and how these natural disasters will be made worse by climate change.

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